Electromagnetic (EM) geophysical surveying involves imparting an electric field or a magnetic field into subsurface Earth formations and measuring electric and/or magnetic field amplitudes by measuring voltage differences induced in electrodes, antennas and/or interrogating magnetometers. The electric and/or magnetic fields are induced in response to the electric and/or magnetic field imparted into the Earth's subsurface, and inferences about the spatial distribution of conductivity of the Earth's subsurface are made from recordings of the induced electric and/or magnetic fields. Such spatial distributions may be used to assist in the location and recovery of hydrocarbons, such as oil or natural gas.
Methods for electromagnetic surveying have been adapted for use in marine environments. The electric and/or magnetic fields may be induced by a electromagnetic source located in a body of water, for example, towed behind a survey vessel. Marine sensors have been devised for detecting electric and/or magnetic field signals resulting from imparting electric and/or magnetic fields into formations below the bottom of a body of water. Such marine sensors may be located in relatively fixed positions at or near the bottom of the body of water, and systems with towed electromagnetic receivers have also been devised. Methods for analyzing the data from the sensors to provide reliable estimates of spatial distributions of conductivity of the Earth's subsurface would be useful.
Note that the figures provided herewith are not necessarily to scale. They are provided for purposes of illustration to ease in the understanding of the presently-disclosed invention.